The present invention relates to a reference clock signal generation circuit, a power supply circuit, a driver circuit, and an electro-optical device.
When driving an electro-optical device such as a liquid crystal display panel, it is necessary to generate various power supply voltages depending on the material for the electro-optical element and the drive method. The power supply voltages are generated by a power supply circuit. The power supply circuit generates the power supply voltages by raising or lowering a system power supply voltage.
The power supply circuit may include a charge-pump circuit which raises or lowers voltage by a charge-pump operation. The charge-pump circuit can efficiently generate a voltage raised or lowered in the positive or negative direction at a low power consumption by the charge-pump operation using a switch device. The charge-pump circuit requires a switch control signal (reference clock signal, step-up clock signal, or step-down clock signal) for controlling the switch device. The frequency of the switch control signal is generally variable, and the output capability of the charge-pump circuit can be increased by increasing the frequency of the switch control signal. However, power consumption is increased by increasing the frequency of the switch control signal.
JP-A-2003-102165 discloses technology relating to a power supply circuit including a charge-pump circuit which allows quick power-on startup and achieves an increase in efficiency and a reduction in power consumption, for example.
In the technology disclosed in JP-A-2003-102165, a circuit which supplies voltage to a controller which generates a switch control signal is switched from a series regulator to a charge-pump circuit in a start period immediately after providing a system power supply and the subsequent operation period.
The specification of a power supply circuit including a charge-pump circuit generally provides a long power-on start period taking the load, manufacturing variation, and the like into consideration. The output capability of the power supply circuit is necessary in the start period. If the charge-pump circuit is operated in the subsequent operation period using the frequency of the switch control signal used in the start period, unnecessary power consumption occurs. As a result, when the power supply circuit is provided in a battery-driven electronic instrument, the battery life is decreased.
When a high output capability is unnecessary for the power supply circuit in the operation period, the user must set the frequency of the switch control signal in the start period taking into consideration the period provided in the specification, and set the frequency of the switch control signal in the operation period at a lower frequency. Therefore, the size of firmware provided by the user or the like for controlling the switch control signal is increased.
Moreover, the actual start period of the power supply circuit is unknown when allowing the user to set the frequency of the switch control signal, whereby it may be difficult to determine whether a malfunction is caused by the user's setting or by a malfunction of the power supply circuit.